Commonly, the electrodes in double ended arc discharge lamps are positioned in opposite ends of a glass tube leaving a tip portions exposed. One end of the electrode is held fixed, and the opposite end of the tube fixed The tube is then heated and collapsed around each electrode root to hold and seal the electrodes in place. By melting and then pressing a portion of the intermediate envelope material the two fixed pieces retain a relative registration. In fact there may be some motion of the electrode in the melted glass, particularly in lamps having extended length seals. During release and subsequent cooling there may be further relative motion between the captured electrode and the envelope volume. The electrode and the envelope are then not correctly aligned. Since there are two such electrodes, there may be two such misaligning motions. The result is then a pair of electrodes that are either not coaxial, or do not have the expected gap separation. The misaligned, or misgapped electrodes cause the arc to be misplaced which then affects the lamp optics and lamp life. There is then a need to accurately position, and gap arc discharge electrodes.
In a similar fashion, each electrode is normally positioned axially by placing the electrode a proper extent within the envelope tube. Again, the electrode may not be positioned along the axis correctly, or the axial positioning may be lost during the sealing process. Change in the axial gap separating the electrodes alters the starting, and operation conditions of an arc lamp. The arc may then require more or less power, or run at a lower or higher temperature than anticipated. There is then a need to accurately gap arc lamp electrodes.
For side by side, parallel arc discharge electrodes, the electrode pair are normally held in a vise like grip while the envelope is sealed around the electrode roots. The electrode tips, being loose ends, may be spread apart, or drawn together by the sealing process. Similarly, the electrodes may be mispositioned during the either the set up or the sealing steps. There is then a corresponding need to accurately align and gap parallel positioned arc lamp electrodes.
Examples of the prior methods of sealing discharge electrodes may be seen in U.S. Pat. No. 4,376,906 issued Mar. 15, 1983 to Imre Szilagyi, and in A New Low Wattage Metal Halide Lamp Process, Journal of the Illuminating Engineering Society, Fall 1985, page 109.